Ablation is the removal of material from the surface of an object by vaporization, chipping, or other erosive processes. The term “ablation” is often used in the context of laser ablation, the process of removing material by irradiating it with a laser beam. At lower laser energy densities, the material is heated by the absorbed laser energy and melts, evaporates and/or sublimates. At high energy densities, the material is typically converted to gas or plasma and expands away from the surface. As a result, small fragments of the material in the form of gases, small liquid and/or solid droplets or particles are freed from the material and either carried away by a gas stream or re-deposited on a nearby surface.
Common parameters of the ablation process include (i) laser beam wavelength, (ii) laser pulse duration and (iii) laser beam fluence. Laser beam wavelength is an important factor because ablation requires sufficient absorption of the laser light by the material. Absorption wavelength characteristics are material-specific. Laser pulse duration is also an important parameter, as the mechanisms of ablation can vary substantially depending on the pulse length. Common pulse regimes include ultra-short (on the order of 10's of fsec to about 10 psec), very short (10 psec-1 nsec), short (1-200 nsec), long (1 μsec-1 msec) and continuous-wave (CW). Laser beam fluence refers to the measure of energy per unit area and is usually measured in J/cm2. The higher the fluence, the more “cutting ability” a laser beam has. This parameter is particularly important because the laser beam fluence must exceed the specific threshold fluence value, Fth, of the material that the laser beam is going into for the laser to actually initiate the ablation process and remove material. Laser beam with a fluence below the Fth threshold value will increase a material's temperature, but will not ablate it. Threshold fluence values are material-specific, wavelength-specific and pulse duration-specific.
Laser ablation is thus greatly affected by the nature of the material and its ability to absorb energy, requiring that at the wavelength of the laser the material has sufficient absorption to enable ablation. The depth over which the laser energy is absorbed, and thus the amount of material removed by a single laser pulse, depends on the material's optical properties at the laser wavelength. Laser pulses can vary over a very wide range of durations (milliseconds to femtoseconds) and fluences and can be precisely controlled.
Thus, laser ablation can be very valuable for both research and industrial applications. Laser ablation is often employed for precise material removal in the fabrication of advanced devices at the scale between microns and hundreds of microns and even at the scale of hundreds of centimeters, e.g., in case of solar panel fabrication. Both direct-write and mask-projection techniques are used, and laser wavelength is selected to be compatible with the materials being processed.
One important application of laser ablation is micromachining and, in particular, the drilling of small orifices having diameters of a size impossible to achieve using mechanical methods. Micromachining is especially useful for working with materials and devices incompatible with mechanical machining. For example, drilling one or more orifices with diameters between 10 and 100 μm in medical devices, such as balloon catheters, cannot be accomplished using convention mechanical machining. A balloon catheter is a type of flexible catheter with an inflatable “balloon” at its tip typically made of a thin (10-200 μm wall thickness) polymer material that is not conducive to mechanical machining. Balloon catheters are used during a catheterization procedure to enlarge a narrow opening or passage within the body and are often employed with drug-eluting balloons or embolic protection filters and used in medical procedures such as angioplasty, kyphoplasty and vertebroplasty.
The balloon itself may be any suitable shape, including without limitation, conical, spherical, cylindrical, zig-zag, tubular, curved, or bent. Some exemplary conventional balloon shapes are shown in FIG. 1. The diameter and length of a catheter balloon may range from a few millimeters to several centimeters, depending on the application. The wall thickness of the balloon typically ranges from 10 to 200 μm. The examples shown in FIG. 1 illustrate the broad spectrum of possible configurations.
In some applications, catheter balloons may be used to dispense and/or collect liquid, in which case one or more orifices must be provided along the body of the balloon. FIG. 2 illustrates a prior art conical balloon having multiple orifices uniformly positioned about the left side of the balloon. FIG. 3 shows a prior art cylindrical balloon 20 having multiple orifices 22 uniformly positioned about a middle portion of the balloon. The diameter of these orifices may range from about 5 to about 200 μm. It is known among those having ordinary skill in mechanical engineering and/or fluid dynamics that the flow rate of a liquid through an orifice can be determined and/or dictated by the size or diameter of the orifice when all other parameters are held constant. Accordingly, the amount of fluid needed in a particular medical application can be precisely administered and controlled by using a balloon that has precisely-manufactured orifices. It will be understood by a person of ordinary skill in the art that manufacturing such orifices in these catheter balloons, or any other device made of “soft” material, to the exact and required dimensions is virtually impossible to accomplish using mechanical machining techniques today.
Alternatively, laser ablation may be employed for precisely drilling orifices in thin flexible materials. Two primary laser ablation techniques useful for drilling applications include mask-projection and beam direct-write. Common parameters of the laser drilling process include (i) laser wavelength, (ii) laser pulse energy, (iii) laser pulse duration, (iv) laser pulse repetition rate, (v) the number of laser pulses delivered, (vi) laser spot size and shape as delivered to the work piece, (vii) laser beam energy density as delivered to the work piece and (viii) the path and velocity of the scanning beam on the work piece. Common pulse regimes include ultra-short (10's of fsec-10 psec), very short (10 psec-1 nsec), short (1-200 nsec), long (1 μsec-1 msec) and continuous-wave (CW).
In conventional laser ablation methods and systems, the orifices formed in a work piece by laser drilling are generally not exactly cylindrical in their cross-section. As shown in FIG. 4, an orifice (10) is characterized by an entrance diameter, Dentrance, located along an outside surface (12) of a wall (11) a work piece and an exit diameter, Dexit, located along an inside surface (13) of the wall (11), wherein Dentrance and Dexit are different. As a result of this difference, a taper angle is formed, as shown in FIG. 4. For a given set of operating parameters of a laser drilling system and a given material, Dentrance and the taper angle are usually defined, while Dexit will depend on the thickness, w, of the wall (11) of the work piece. The flow rate of liquid through the orifice (10) is mostly dependent upon Dexit. Thus, precisely controlling the amount of liquid that flows through orifice (10) requires controlling the exit diameter, Dexit, to the exact, called-for dimension.
It is also well-known to those of ordinary skill in the art that current manufacturing methods (e.g., extrusion, thermal-forming and injection molding) for fabricating soft-material devices, such as catheter balloons, are unable to precisely and consistently control the wall thickness, w, of such devices, either (i) when transitioning between differently-sized sections of the same device, (ii) from one device to another in the same manufacturing run and/or (iii) between different manufacturing runs. By way of example, FIG. 5 illustrates a catheter balloon having uniform wall thickness, while FIG. 6 shows a catheter balloon having varying wall thickness. Because the size of the exit diameter, Dexit, depends upon the thickness, w, of the wall of a work piece, as stated above, it is very difficult to precisely machine the exit diameter, Dexit, in a catheter balloon, or other soft-material device, having a non-uniform wall using pre-selected drilling parameters. By selecting such parameters ahead of time, the laser drilling operation cannot account for the numerous adjustments needed to be made in response to the varying thickness of the wall of the balloon. Accordingly, there is a need in the art for methods and systems of drilling precisely-controlled orifices in a soft-material wall of non-uniform thickness.